Lubricant composition

ABSTRACT

Provided by the present invention is a lubricating oil composition showing, though containing ZnDTP as an abrasion resistant agent, a low friction coefficient when used as a lubricating oil for a low friction sliding material, that is, a specific lubricating oil composition which is prepared by blending a lubricant base oil with (A) a specific alkali earth metal-containing cleaning agent, (B) a specific boron-non-containing succinimide base ashless dispersant, (C) a zinc dialkyldithiophosphate and (D) a specific sulfur base compound and which is used for a low friction sliding material.

TECHNICAL FIELD

The present invention relates to a lubricating oil composition, more specifically to a lubricating oil composition showing, though containing a zinc dialkyldithiophosphate, a low frictional coefficient when used as a lubricating oil for a low friction sliding material.

BACKGROUND ART

In recent years, it is important to meet environmental concerns in various fields, and technical development on energy saving and a reduction in a discharge amount of carbon dioxide is promoted. In a case of, for example, automobiles, an improvement in a fuel consumption-saving performance is one of issues, and technical development of lubricating oils and sliding materials becomes important.

In respect to lubricating oils, various base oils and additives have so far been developed for the purpose of enhancing various performances. For example, performances required to engine oils include an appropriate viscosity characteristic, an oxidation stability, a clean dispersibility, an abrasion preventing property, a bubbling preventing property and the like, and the above performances are attempted to be elevated by combination of various base oils and additives. In particular, zinc dithiophosphate (ZnDTP) is excellent as an additive for abrasion resistance and therefore is used well as an additive for engine oils.

On the other hand, in respect to the sliding materials, materials forming a hard film such as a TiN film, a CrN film and the like which contribute to a rise in an abrasion resistance are known as materials for parts which are exposed to severe friction and abrasion environment (for example, a sliding part of an engine). Further, it is known that a friction coefficient can be reduced in the air under the absence of a lubricating oil by making use of a diamond-like carbon (DLC) film, and materials having a DLC film (hereinafter referred to as a DLC material) are expected as a low friction sliding material.

However, a friction-reducing effect of a DLC material is small under the presence of a lubricating oil in a certain case, and in the above case, a fuel consumption-saving effect is less liable to be obtained. Accordingly, development of a lubricating oil composition for a low friction sliding material such as DLC materials and the like has so far been carried out.

A lubricating oil composition which contains an ether base ashless friction-reducing agent and which is used for a low friction sliding member is disclosed in, for example, a patent document 1. Disclosed in patent documents 2 and 3 are techniques in which lubricating oil compositions containing fatty acid ester base ashless friction-controlling agents and aliphatic amine base ashless friction-controlling agents are used for a sliding face between a DLC member and an iron base member and a sliding face between a DLC member and an aluminum alloy member. Disclosed in a patent document 4 is a technique in which a low friction agent composition containing an oxygen-containing organic compound and an aliphatic amine base compound is used in a low friction sliding mechanism having a DLC coating sliding member.

As described above, lubricating oil compositions for low friction sliding materials have been developed, but even in a case in which the above techniques are applied, the friction coefficient tends to grow larger when ZnDTP is blended in order to enhance further the abrasion resistance and the like. Accordingly, when a purpose is to obtain a lubricating oil which is excellent in various performances and well balanced, required is a lubricating oil composition which shows, though containing ZnDTP, a low friction coefficient when used as a lubricating oil for a low friction sliding material.

PRIOR ART DOCUMENTS Patent Documents

-   Patent document 1: JP 2006 36850A -   Patent document 2: JP 2003 238982A -   Patent document 3: JP 2004 155891A -   Patent document 4: JP 2005 98495A

OUTLINE OF THE INVENTION Problems to be Solved by the Invention

The present invention has been made in light of the situations, and an object of the present invention is to provide a lubricating oil composition showing, though containing ZnDTP as an abrasion resistant agent, a low friction coefficient when used as a lubricating oil for a low friction sliding material, that is, a lubricating oil composition for a low friction sliding material in which an abrasion resistance is consistent with a low frictional property.

Means for Solving the Problems

Researches repeated intensively by the present inventors have resulted in finding that the problems are solved by blending with specific additives in specific amounts. The present invention has been completed based on the above knowledge.

That is, the present invention provides:

-   1. a lubricating oil composition used for a low friction sliding     material, which is prepared by blending a lubricant base oil     with (A) an alkali earth metal salicylate and/or alkali earth metal     sulfonate base cleaning agents, (B) a boron-non-containing     succinimide base ashless dispersant having an alkenyl group or an     alkyl group having a number average molecular weight of 500 to     4000, (C) a zinc dialkyldithiophosphate and (D) a sulfur base     compound selected from polysulfide compounds, sulfurized oil & fats,     olefin sulfides, thiophosphoric esters, thiophosphorous esters and     amine salts of the above esters, wherein blending amounts thereof     based on a whole amount of the composition are 0.05 to 0.25% by mass     of the component (A) in terms of alkali earth metal concentration,     0.03 to 0.50% by mass of the component (B) in terms of nitrogen     concentration, 0.01 to 0.12% by mass of the component (C) in terms     of phosphorus concentration, 0.02 to 2.0% by mass of the     component (D) in terms of sulfur concentration, 0.10% by mass or     less of an alkali earth metal phenate base cleaning agent in terms     of alkali earth metal concentration and 0.04% by mass or less of     boron-containing succinimide in terms of nitrogen concentration, -   2. the lubricating oil composition according to the item 1, wherein     a blending amount of a zinc dialkyldithiophosphate is 0.03 to 0.10%     by mass in terms of phosphorus concentration, -   3. the lubricating oil composition according to the item 1, wherein     a blending amount of the boron-non-containing succinimide base     ashless dispersant having an alkenyl group or an alkyl group having     a number average molecular weight of 500 to 4000 is 0.05 to 0.30% by     mass in terms of nitrogen concentration, -   4. the lubricating oil composition according to the item 1, wherein     the sulfur base compound is a compound represented by a general     formula (I):

R¹—S_(n)—R²   (I)

(wherein “n” is an integer selected from 1 to 4; R¹ and R² each represent independently an alkyl group or an aralkyl group),

-   5. the lubricating oil composition according to the item 4, wherein     “n” in the general formula (I) is 2 or 3, and -   6. the lubricating oil composition according to the item 1, wherein     the low friction sliding material is a material having a     diamond-like carbon (DLC) film.

Effect by the Invention

According to the present invention, capable of being provided is a lubricating oil composition showing, though containing ZnDTP as an abrasion resistant agent, a low friction coefficient when used as a lubricating oil for a low friction sliding material.

MODE FOR CARRYING OUT THE INVENTION

The lubricating oil composition of the present invention is prepared by blending a lubricant base oil with (A) an alkali earth metal salicylate and/or alkali earth metal sulfonate base cleaning agents, (B) a boron-non-containing succinimide base ashless dispersant having an alkenyl group or an alkyl group having a number average molecular weight of 500 to 4000, (C) a zinc dialkyldithiophosphate and (D) a sulfur base compound selected from polysulfide compounds, sulfurized oil & fats, olefin sulfides, thiophosphoric esters, thiophosphorous esters and amine salts of the esters as essential additives in specific amounts, and it is used as a lubricating oil for a low friction sliding material.

The lubricant base oil used in the present invention shall not specifically be restricted, and there can be used base oils suitably selected from publicly known mineral base oils and synthetic base oils which have so far been used.

In this regard, capable of being listed as the mineral base oils are, for example, distillate oils obtained by distilling paraffin base crude oils, intermediate base crude oils or naphthene base crude oils at atmospheric pressure or subjecting residual oils obtained by atmospheric distillation to distillation under reduced pressure, or refined oils obtained by refining the above distillate oils according to an ordinary method, for example, solvent-refined oils, hydrogenation-refined oils, dewaxing-treated oils, white clay-treated oils and the like.

On the other hand, poly(α-olefins) which are α-olefin oligomers having 8 to 14 carbon atoms, polybutene, polyol esters, alkylbenzenes and the like can be listed as the synthetic oils.

In the present invention, the mineral oils may be used alone or in combination of two or more kinds thereof as the base oil. Also, the synthetic oils may be used alone or in combination of two or more kinds thereof. Further, at least one mineral oil and at least one synthetic oil may be used in combination.

It is advantageous that the base oil has a kinetic viscosity of usually 2 to 50 mm²/s, preferably 3 to 30 mm²/s and particularly preferably 3 to 15 mm²/s at 100° C. If the kinetic viscosity at 100° C. is 2 mm²/s or more, the vaporization loss is small. On the other hand, if it is 50 mm²/s or less, the power loss brought about by the viscous resistance is inhibited, and the fuel consumption-improving effect is exerted well.

Further, the above base oil has a viscosity index of preferably 60 or more, more preferably 70 or more and particularly preferably 80 or more. If the viscosity index is 60 or more, a viscosity change of the base oil brought about by temperature is small, and the stable lubricating performances are exerted.

Compounds which have so far been publicly known as cleaning agents such as compounds having an alkyl group can be used as alkali earth metal salicylate and alkali earth metal sulfonate which are used as the cleaning agents of the component (A). The alkali earth metal includes calcium, magnesium and the like, and calcium is particularly preferred.

Any of neutral salts, basic salts and perbasic salts can be used as the cleaning agents, and they may be used alone or in combination of two or more kinds thereof.

A whole base number of the cleaning agent can optionally be selected according to the required performances of the lubricating oil composition. It is usually 0 to 500 mg KOH/g, preferably 50 to 400 mg KOH/g measured by a perchloric acid method. A blending amount of the cleaning agent based on a whole amount of the compositions is 0.05 to 0.25% by mass, preferably 0.06 to 0.25% by mass and more preferably 0.1 to 0.22% by mass in terms of alkali earth metal concentration. If the alkali earth metal concentration exceeds 0.25% by mass, it is difficult to reduce the friction coefficient.

Compounds other than the component (A) are also publicly known as the alkali earth metal-containing cleaning agent, and they include, for example, alkali earth metal phenate base cleaning agents. However, if the alkali earth metal phenate base cleaning agent is blended, it is difficult to reduce the friction coefficient, and therefore use thereof has to be restricted in the present invention. A content thereof based on a whole amount of the compositions is 0.1% by mass or less, preferably 0.05% by mass or less in terms of alkali earth metal concentration, and it is particularly preferably not added at all.

In the present invention, the boron-non-containing succinimide compound (hereinafter the succinimide compound shall be referred to as “the succinimide compound of the present invention”) having an alkenyl group or an alkyl group of a number average molecular weight of 500 to 4000 is blended as the ashless dispersant of the component (B). If a number average molecular weight of an alkenyl group or an alkyl group is 500 or more, the good cleaning effect is exerted, and if it is 4000 or less, the low temperature fluidity is good.

The succinimide compound of the present invention includes, for example, a compound represented by the following a general formula (II) or a general formula (III):

R³ to R⁵ in the general formulae (II) and (III) each represent independently an alkenyl group or an alkyl group of a number average molecular weight of 500 to 4000, and “m” represents an integer of 1 to 5. If “m” is an integer of 1 to 5, the cleaning property is enhanced, and “m” is more preferably an integer of 2 to 4.

R³ to R⁵ in the general formulae (II) and (III) include, for example, an alkenyl group or an alkyl group originating in polybutene and the like obtained by polymerizing high purity isobutene or a mixture of 1-butene and isobutene by a boron fluoride base catalyst or an aluminum chloride base catalyst.

A production method of the succinimide compound of the present invention shall not specifically be restricted, and it can be obtained, for example, by reacting butenylsuccinic acid obtained by reacting polybutene or chlorinated polybutene with maleic anhydride at 100 to 200° C. with polyamine such as diethylenetriamine, triethylenetetraamine, tetraethylenepentaamine, pentaethylenehexamine and the like. The polybutene and the like which are precursors of the alkenyl group or the alkyl group are advantageously used after trace amounts of remaining fluorine substances and chlorine substances originating in a catalyst used in a production step have been removed up to usually 50 ppm by mass or less, preferably 10 ppm by mass or less and particularly preferably 1 ppm by mass or less by a suitable method such as an adsorbing method, sufficient washing with water and the like.

In the present invention, the boron-non-containing succinimide compound is used as described above. The term “boron-non-containing” means that a succinimide compound treated by a boron compound is excluded from the component (B) in the present application. That is, a boron-containing polybutenylsuccinimide compound obtained by reacting a polybutenylsuccinimide compound with a boron compound such as boric acid, borates, boric esters and the like to neutralize a part or all of a remaining amino group and/or imino group is known as an ashless dispersant. However, blending of the above boron-containing polybutenylsuccinimide compound makes it difficult to reduce the abrasion when it is used as a lubricating oil for a low friction sliding material.

In the lubricating oil composition of the present invention, the succinimide compound in the present invention may be used alone or in combination of two or more kinds thereof. A blending amount of the succinimide compound in the present invention is 0.03 to 0.50% by mass, preferably 0.05 to 0.30% by mass in terms of nitrogen concentration based on a whole amount of the compositions. If the nitrogen concentration falls in the range, the effects preferred in terms of a balance between the cleaning property, the resistance in emulsifying and the economical efficiency are obtained.

As described above, blending of the boron-non-containing succinimide compound makes it difficult to reduce the friction coefficient, and therefore use thereof has to be limited in the present invention. A content thereof is 0.04% by mass or less, preferably 0.02% by mass or less in terms of nitrogen concentration based on a whole amount of the compositions, and it is particularly preferably not added at all.

The zinc dialkyldithiophosphate of the component (C) includes, for example, a compound represented by a general formula (IV):

In the general formula (IV), R⁶ to R⁹ each represent independently an alkyl group, and an alkyl group having 1 to 24 carbon atoms is preferably used.

The alkyl group having 1 to 24 carbon atoms may be any of linear, branched and cyclic groups and includes, to be specific, methyl, ethyl and in addition thereto, various kinds each including isomers, such as propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl, undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl, hexadecyl, heptadecyl, octadecyl, nonadecyl, icosyl, henicosyl, docosyl, tricosyl and tetracosyl, or cyclopentyl, cyclohexyl, cycloheptyl and alkyl-substituted groups thereof.

The specific examples of the zinc dialkyldithiophosphate represented by the general formula (IV) include, for example, zinc diisopropyldithiophosphate, zinc diisobutyldithio phosphate, zinc di-sec-butyldithiophosphate, zinc di-sec-pentyldithiophosphate, zinc di-n-hexyldithiophosphate, zinc di-sec-hexyldithiophosphate, zinc di-octyldithiophosphate, zinc di-2-ethylhexyldithiophosphate, zinc di-n-decyldithio phosphate, zinc di-n-dodecyldithiophosphate, zinc diisotridecyldithiophosphate and the like. Among them, zinc di-sec-alkyldithiophosphates are suited in terms of an enhancing effect of the abrasion resistance.

In the lubricating oil composition of the present invention, the zinc dialkyldithiophosphate may be blended alone or in combination of two or more kinds thereof. A blending amount of the zinc dialkyldithiophosphate based on a whole amount of the compositions is 0.01 to 0.12% by mass, preferably 0.03 to 0.10% by mass in terms of phosphorus concentration. The abrasion resistance and the high temperature cleaning property are enhancing by blending the zinc dialkyldithiophosphate, but if it exceeds 0.12% by mass in terms of phosphorus concentration, it is difficult to reduce the abrasion coefficient.

The sulfur base compound of the component (D) is a compound selected from polysulfide compounds, sulfurized oil & fats, olefin sulfides, thiophosphoric esters, thiophosphorous esters and amine salts of the esters. Among them, the polysulfide compounds are preferred, and a compound represented by the general formula (I) is particularly preferred:

R¹—S_(n)—R²   (I)

In the general formula (I), “n” is an integer selected from 1 to 4, and “n” is particularly preferably 2 or 3. R¹ and R² each represent independently an alkyl group or an aralkyl group. R¹ and R² are preferably a group having 4 to 24 carbon atoms, more preferably a group having 8 to 18 carbon atoms.

In the present invention, the sulfur base compound may be used alone or in combination of two or more kinds thereof as the base oil. A blending amount of the sulfur base compound based on a whole amount of the compositions is 0.02 to 2.0% by mass, preferably 0.02 to 1.0% by mass in terms of sulfur concentration. Either when the blending amount is less than 0.02% by mass or when it exceeds 2.0% by mass, it is difficult to reduce the friction coefficient.

The lubrication oil composition of the present invention may be blended with additives which have so far been publicly known as long as the effects of the present invention are not damaged, and they include, for example, friction-reducing agents, viscosity index-improving agents, pour point depressants, antioxidants, rust preventives and the like.

The friction-reducing agents include ashless friction-reducing agents such as fatty acid esters, aliphatic amines, higher alcohols and the like. Capable of being shown as the examples of the viscosity index-improving agents are, to be specific, so-called non-dispersion type viscosity index-improving agents such as copolymers according to various methacrylic esters or optional combinations thereof and hydrogenated products thereof and so-called dispersion type viscosity index-improving agents obtained by copolymerizing various methacrylic esters including nitrogen compounds. Also, capable of being shown as the examples thereof are non-dispersion type or dispersion type ethylene-α-olefin copolymers (the α-olefin includes, for example, propylene, 1-butene, 1-pentene and the like) and hydrogenated products thereof, polyisobutylene and hydrogenated products thereof, styrene-diene hydrogenated copolymers, styrene-maleic anhydride ester copolymers, polyalkylstyrenes and the like. The molecular weights of the above viscosity index-improving agents have to be selected considering the shearing stability. To be specific, a number average molecular weight of the above viscosity index-improving agents is 5000 to 1000000, preferably 100000 to 800000 in a case of, for example, the dispersion type or non-dispersion type polymethacrylates; 800 to 5000 in a case of polyisobutylene or the hydrogenated products thereof; and 800 to 300000, preferably 10000 to 200000 in a case of the ethylene-α-olefin copolymers and the hydrogenated products thereof. Also, the viscosity index-improving agents can be added alone or in optional combination of plural kinds thereof, and a content thereof is usually 0.1 to 40.0% by mass based on a whole amount of the lubricating oil composition. The pour point depressants include, for example, polymethacrylates.

The antioxidant includes phenol base antioxidants and amine base antioxidants. The phenol base antioxidants include, for example, 4,4′-methylenebis(2,6-di-t-butylphenol); 4,4′-bis(2,6-di-t-butylphenol); 4,4′-bis(2-methyl-6-t-butylphenol); 2,2′-methylenebis(4-ethyl-6-t-butylphenol); 2,2′-methylenebis(4-methyl-6-t-butylphenol); 4,4′-butylidenebis(3-methyl-6-t-butylphenol); 4,4′-isopropylidenebis(2,6-di-t-butylphenol); 2,2′-methylenebis(4-methyl-6-nonylphenol); 2,2′-isobutylidenebis(4,6-dimethylphenol); 2,2′-methylenebis(4-methyl-6-cyclohexylphenol); 2,6-di-t-butyl-4-methylphenol; 2,6-di-t-butyl-4-ethylphenol; 2,4-dimethyl-6-t-butylphenol; 2,6-di-t-amyl-p-cresol; 2,6-di-t-butyl-4-(N,N′-dimethylaminomethylphenol); 4,4′-thiobis(2-methyl-6-t-butylphenol); 4,4′-thiobis(3-methyl-6-t-butylphenol); 2,2′-thiobis(4-methyl-6-t-butylphenol); bis(3-methyl-4-hydroxy-5-t-butylbenzyl)sulfide; bis(3,5-di-t-butyl-4-hydroxybenzyl)sulfide; n-octadecyl-3-(4-hydroxy-3,5-di-t-butylphenyl)propionate; 2,2′-thio[diethyl-bis-3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate] and the like. Among them, bisphenol base antioxidants and ester group-containing phenol base antioxidants are particularly suited.

The amine base antioxidants include, for example, monoalkyldiphenylamines such as monooctyldiphenylamine, monononyldiphenylamine and the like; dialkyldiphenylamines such as 4,4′-dibutyldiphenylamine, 4,4′-dipentyldiphenylamine, 4,4′-dihexyldiphenylamine, 4,4′-diheptyldiphenylamine, 4,4′-diocyldiphenylamine, 4,4′-dinonyldiphenylamine and the like; polyalkyldiphenylamines such as tetrabutyldiphenylamine, tetrahexyldiphenylamine, tetraoctyldiphenylamine, tetranonyldiphenylamine and the like; naphthyl amine base antioxidants, to be specific, α-naphthylamine, phenyl-α-naphthylamine; and alkyl-substituted phenyl-α-naphthylamines such as butylphenyl-α-naphthylamine, pentylphenyl-α-naphthylamine, hexylphenyl-α-naphthylamine, heptylphenyl-α-naphthylamine, octylphenyl-α-naphthylamine, nonylphenyl-α-naphthylamine and the like. Among them, the dialkyldiphenylamine base antioxidants and the naphthylamine base antioxidants are suited.

The rust preventives include alkylbenzenesulfonates, dinonylnaphthalenesulfonates, alkenylsuccinic esters, polyhydric alcohol esters and the like.

The lubricating oil composition of the present invention is applied to a sliding face having a low friction sliding material and can provide it with an excellent low friction property, and particularly when applied to internal combustion engines, they can be provided with a fuel consumption-saving effect.

The sliding face having a low friction sliding material is particularly preferably a sliding face having a DLC material as the low friction sliding material at one side thereof. In this case, the opposite material shall not specifically be restricted, and a sliding face between, for example, the above DLC material and the iron base material and a sliding face between the DLC material and the aluminum alloy material can be listed.

In this connection, the DLC material has a DLC film on a surface. The DLC material constituting the above film is an amorphous material constituted principally from a carbon element, and a bonding form of carbons themselves comprises both of a diamond structure (SP₃ bond) and a graphite bond (SP₂ bond). To be specific, it includes a-C (amorphous carbon) comprising only a carbon element, a-C:H (hydrogen amorphous carbon) containing hydrogen and MeC containing partially a metal element such as titanium (Ti), molybdenum (Mo) and the like.

On the other hand, carburized steel SCM420, SCr420 (JIS) and the like can be listed as the iron base material. A hypoeutectic aluminum alloy containing 4 to 20% by mass of silicon and 1.0 to 5.0% by mass of copper or a hypereutectic aluminum alloy is preferably used as the aluminum alloy material. To be specific, AC2A, AC8A, ADC12, ADC14 (JIS) and the like can be listed.

Also, each surface roughness of the DLC material, the iron base material or the DLC material and the aluminum alloy material each is suitably 0.1 μm or less in terms of an arithmetic average roughness Ra from the viewpoint of a stability of sliding. If it is 0.1 μm or less, local scuffing is less liable to be formed, and the friction coefficient can be inhibited from growing larger. Further, the DLC material has preferably a surface hardness of HV 1000 to 3500 in terms of a micro-Vickers hardness (98 mN load) and a thickness of 0.3 to 2.0 μm.

On the other hand, the iron base material has preferably a surface hardness of HRC 45 to 60 in terms of a Rockwell hardness (C scale). In this case, a durability of the film can be maintained even on a sliding condition of about 700 MPa under a high face pressure as is the case with a cam follower member, and therefore it is effective.

Also, the aluminum alloy material has preferably a surface hardness of HB 80 to 130 in terms of a Brinell hardness.

If a surface hardness and a thickness of the DLC material fall in the ranges, abrasion and peeling are inhibited. Further, if a surface hardness of the iron base material is HRC 45 or more, it can be inhibited from buckling and peeling under a high face pressure. On the other hand, if a surface hardness of the aluminum alloy material falls in the range, the aluminum alloy material is inhibited from abrading.

The sliding part to which the lubricating oil composition of the present invention is applied shall not specifically be restricted as long as it is a surface in which two metal surfaces are brought into contact and in which at least one of them has a low friction sliding material, and a sliding part of an internal combustion engine can be preferably listed. In the above case, the very excellent low frictional property as compared with ever is obtained, and the fuel consumption-saving effect is exerted, so that it is effective. The DLC member includes, for example, discoid shims and lifter crestal planes each obtained by coating DLC on a base plate of a steel material, and the iron base material includes low alloy chilled cast irons, carburized steels or quenched and tempered carbon steels and cam lobes prepared by using materials obtained according to optional combinations thereof.

EXAMPLES

Hereinafter, the present invention shall be explained in further details with reference to examples, but the present invention shall by no means be restricted by these examples.

Examples 1 to 6 and Comparative Examples 1 to 5

The lubricating oil compositions of the invention 1 each having compositions shown in Table 1 were prepared and subjected to a frictional characteristic test shown below to determine a friction coefficient. The results thereof are shown in Table 2.

<Frictional Characteristic Test>

A reciprocating friction test equipment (SRV reciprocating friction test equipment manufactured by Optimal Inc.) was used to measure the friction coefficient by the following method.

A disc (φ24 mm×7.9 mm) on which DLC was coated was used as a test piece, and several droplets of the sample oil (lubricating oil composition) were dropped thereon. The friction coefficient was determined on the conditions of a load of 400N, an amplitude of 1.5 mm, a frequency of 50 Hz and a temperature of 80° C. in a state in which a cylinder (φ15 mm×22 mm) made of SCM420 was set on an upper part of the disc.

TABLE 1 Example Comparative Example 1 2 3 4 5 6 1 2 3 4 5 Lubricant base oil Bal- Bal- Bal- Bal- Bal- Bal- Bal- Bal- Bal- Bal- Bal- ance ance ance ance ance ance ance ance ance ance ance (A) Metal Ca sulfonate (1) — — 0.50 0.50 0.50 0.50 — 0.50 0.50 0.50 0.50 base Ca sulfonate (2) — 1.70 — — — — — — — — 2.55 cleaning Ca salicylate (1) — — 1.54 1.54 1.54 1.54 — 1.54 1.54 1.54 1.54 agent Ca salicylate (2) 8.70 — — — — — — — — — — — Ca phenate — — — — — — 2.20 — — — — (B) Ashless Boron-non- 3.62 3.62 3.62 3.62 — 3.62 3.62 — 3.62 3.62 3.62 dispersant containing succinimide (1) Boron-non- — — — — 7.84 — — — — — — containing succinimide (2) — Boron-containing — — — — — — — 6.18 — — — succinimide (C) Zinc Zinc dialkyl- 1.00 1.00 1.00 1.00 1.00 1.22 1.00 1.00 1.22 1.50 1.00 dialkyl- dithiophosphate dithio- (1) phosphate Zinc dialkyl- 0.15 0.15 0.15 0.15 0.15 — 0.15 0.15 — 0.23 0.15 dithiophosphate (2) (D) Sulfur Polysulfide 0.20 0.20 0.20 0.20 0.20 0.20 0.20 0.20 — 0.20 0.20 base compound Other Viscosity index- 3.40 3.40 3.40 3.40 3.40 3.40 3.40 3.40 3.40 3.40 3.40 additives improving agent Pour point 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 depressant Antioxidant (1) 0.80 0.80 0.80 0.80 0.80 0.80 0.80 0.80 0.80 0.80 0.80 Antioxidant (2) 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 Rust preventive 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 Total 100 100 100 100 100 100 100 100 100 100 100 Alkali earth metal amount 0.20 0.20 0.20 0.20 0.20 0.20 0.20 0.20 0.20 0.20 0.31 Amount of N originating in 0.08 0.08 0.08 0.08 0.08 0.08 0.08 0.08 0.08 0.08 0.08 succinimide Amount of P originating in 0.09 0.09 0.09 0.09 0.09 0.10 0.09 0.09 0.10 0.14 0.09 Zinc dialkyldithiophosphate Amount of S originating in 0.04 0.04 0.04 0.04 0.04 0.04 0.04 0.04 0.00 0.04 0.04 sulfur base compound (unit: % by mass)

The respective components used for preparing the lubricating oil compositions are shown below.

-   Lubricant base oil: hydrocracked mineral oil (kinematic viscosity at     100° C.: 4.47 mm²/s) -   Ca sulfonate (1): Ca sulfonate (Ca content: 15.2% by mass) -   Ca sulfonate (2): Ca sulfonate (Ca content: 12.0% by mass) -   Ca salicylate (1): Ca salicylate (Ca content: 7.8% by mass) -   Ca salicylate (2): Ca salicylate (Ca content: 2.3% by mass) -   Ca phenate: Ca phenate (Ca content: 9.25% by mass) -   Boron-non-containing succinimide (1): boron-non-containing     polybutenylsuccinimide (nitrogen content: 2.1% by mass, number     average molecular weight of a polybutenyl group: 1000) represented     by the formula (III) -   Boron-non-containing succinimide (2): boron-non-containing     polybutenylsuccinimide (nitrogen content:0.97% by mass, number     average molecular weight of a polybutenyl group: 1300) represented     by the formula (II) -   Boron-containing succinimide: boron-containing     polybutenylsuccinimide (nitrogen content: 1.23% by mass, number     average molecular weight of a polybutenyl group: 1000) represented     by the formula (III) -   Zinc dialkyldithiophosphate (1): a secondary alkyl type zinc     dialkyldithiophosphate (phosphorus content: 8.2% by mass) -   Zinc dialkyldithiophosphate (2): a primary alkyl type zinc     dialkyldithiophosphate (phosphorus content: 7.4% by mass) -   Sulfur base compound: polysulfide mixture (R—S_(a)—R, R is an alkyl     group having 12 carbon atoms, “a” is 2 or 3, sulfur content: 22.0%     by mass) -   Viscosity index-improving agent: polymethacrylate (weight average     molecular weight Mw: 550,000) -   Pour point depressant: polymethacrylate (weight average molecular     weight Mw: 69,000) -   Antioxidant (1): dialkyldiphenylamine (nitrogen content: 4.62% by     mass) -   Antioxidant (2): 4,4′-methylenebis(2,6-di-tert-butylphenol) -   Rust preventive: N-alkylbenzotriazole

TABLE 2 Example Comparative Example 1 2 3 4 5 6 1 2 3 4 5 DLC DLC DLC DLC DLC W DLC DLC DLC DLC DLC DLC DLC Reciprocating 0.143 0.146 0.142 0.140 0.145 0.146 0.153 0.160 0.159 0.155 0.156 friction test equipment (friction coefficient)

The following discs were used as the disc on which DLC was coated:

-   DLC: DLC containing 20% of hydrogen -   DLC W: DLC containing (tungsten added) 20% of hydrogen

It can be found from the results shown in Table 2 that the compositions prepared in Examples 1 to 6 which are the lubrication oil compositions of the present invention have a low friction coefficient and are excellent. On the other hand, Ca phenate and boron-containing succinimide are blended respectively as the metal base cleaning agent and the ashless dispersant in Comparative Examples 1 and 2, and therefore the friction coefficients are high. In Comparative Example 3, the sulfur base compound is not blended, and therefore the friction coefficient is high. In Comparative Examples 4 and 5, the phosphorus amount or the calcium amount is present to excess, and therefore the friction coefficients are elevated.

INDUSTRIAL APPLICABILITY

The lubricating oil composition of the present invention is applied to a sliding face comprising a low friction sliding material such as a DLC material and can provide it with an excellent low friction characteristic, and particularly when applied to internal combustion engines, they can be provided with a fuel consumption-saving effect. 

1. A lubricating oil composition used for a low friction sliding material, which is prepared by blending a lubricant base oil with (A) an alkali earth metal salicylate base and/or alkali earth metal sulfonate base cleaning agents, (B) a boron-non-containing succinimide base ashless dispersant having an alkenyl group or an alkyl group having a number average molecular weight of 500 to 4000, (C) a zinc dialkyldithiophosphate and (D) a sulfur base compound selected from polysulfide compounds, sulfurized oil & fats, olefin sulfides, thiophosphoric esters, thiophosphorous esters and amine salts of the esters, wherein blending amounts thereof based on a whole amount of the composition are 0.05 to 0.25% by mass of the component (A) in terms of alkali earth metal concentration, 0.03 to 0.50% by mass of the component (B) in terms of nitrogen concentration, 0.01 to 0.12% by mass of the component (C) in terms of phosphorus concentration, 0.02 to 2.0% by mass of the component (D) in terms of sulfur concentration, 0.10% by mass or less of an alkali earth metal phenate base cleaning agent in terms of alkali earth metal concentration and 0.04% by mass or less of boron-containing succinimide in terms of nitrogen concentration.
 2. The lubricating oil composition according to claim 1, wherein the blending amount of a zinc dialkyldithiophosphate is 0.03 to 0.10% by mass in terms of phosphorus concentration.
 3. The lubricating oil composition according to claim 1, wherein the blending amount of the boron-non-containing succinimide base ashless dispersant having an alkenyl group or an alkyl group having a number average molecular weight of 500 to 4000 is 0.05 to 0.30% by mass in terms of nitrogen concentration.
 4. The lubricating oil composition according to claim 1, wherein the sulfur base compound is a compound represented by a general formula (I): R¹—S_(n)—R²   (I) (wherein “n” is an integer selected from 1 to 4; R¹ and R² each represent independently an alkyl group or an aralkyl group).
 5. The lubricating oil composition according to claim 4, wherein “n” in the general formula (I) is 2 or
 3. 6. The lubricating oil composition according to claim 1, wherein the low friction sliding material is a material having a diamond-like carbon (DLC) film. 